Our research interest is in the biochemical and electrophysiological correlates of learning. One, and we believe the fastest, approach to elucidating the bases of learning processes is to study them in simplified, generally invertebrate, systems. In view of this we are studying the process of habituation using the protozoan Stentor coeruleus as a model system. Previously performed work shows this process is similar to metazoan habituation and that it is correlated with a reduction in receptor amplitude. Recent studies indicate that the reduction in receptor potential amplitude is not due to a change in ionic activities but rather results from a change in a cholinergic mechanoreceptor mechanism. The studies proposed in this renewal application are designed to elucidate the biochemical basis of this cholinergic mechanoreceptor through the use of iontophoresis, histochemistry and biochemical assays. Subsequent to the characterization of the cholinergic mechanoreceptor it is proposed to use the same techniques to study which component of the cholinergic receptor mechanism changes during habituation.